A vehicle under body which forms the floor-side framework of the vehicle has a pair of side members, which extend in the front-rear direction of the vehicle, on both sides with respect to the width direction of the vehicle in order to impart impact resistance or the like. Regarding the pair of side members, which extend generally straight along the front-rear direction of the vehicle when viewed from the top, for example, in order to widen the battery installation space, the width of the pair of side members is increased in the width direction of the vehicle.
For example, JP 2014-12524 A describes a structure in which the width along the width direction of the vehicle of a pair of side members, which extend in the front-rear direction and are provided on both sides of the body of an electric car, increases from the front of the vehicle toward the vehicle interior, and a battery pack is disposed in this wide space. Here, a battery cross member mounted with the right and left ends of the pair of side members supports the bottom surface of the battery pack. The battery cross member is composed of a metal thin plate in a lattice shape in which a portion extending in the width direction of the vehicle and a portion extending in the front-rear direction of the vehicle are combined with each other.
When a vehicle receives an impact due to, for example, front collision, the pair of side members extending in the front-rear direction and provided on both sides of the vehicle body deform and, depending on the circumstances, the vehicle interior space may significantly narrow. In particular, if the distance between the pair of side members is increased in the width direction of the vehicle, the side members are curved, being inclined toward the outer side of the vehicle from the front of the vehicle toward the rear of the vehicle when viewed from the top, and may be bent at the inflection point of the curve when receiving an impact. In the side members extending in the front-rear direction of the vehicle, the section including the inclined portion is a floor side member. When a floor side member is referred to as a floor member, a vehicle body structure is required in which, in a pair of floor members, the impact resistance is improved in the inclined portion inclined toward the outer side of the vehicle from the front of the vehicle toward the rear of the vehicle, when viewed from the top.